258 STUDIES IN IMMUNITY. 



sera of guinea-pigs injected with rabbit blood, although in this 

 case the sensitizer appears less powerful. 



2. The sensitizers studied by Bordet cause blood cells or bac- 

 teria to fix alexin. The sensitizers we have just described produce 

 the same phenomenon and differ only in being directed against non- 

 differentiated substances. We have dealt with amorphous chemical 

 substances and hot with morphologically defined elements. 



3. The sensitizer in the serum of rabbits injected with dog 

 blood would seem to act both on the globulin and the albumin of 

 dog serum; the sensitizer in the serum of rabbits injected with cow 

 milk acts on the casein and the lactoglobulin, but not on the lact- 

 albumin. 



4. It is known that the sensitizers of antimicrobial and of hemo- 

 lytic sera generally have the character of marked specificity. We 

 have found this to be true also of the serum of rabbits active against 

 dog serum. On the contrary, the sensitizers of most of the sera 

 we have studied have slight specificity or none at all. Such sera, 

 notably, are those from animals injected with milk, egg white and 

 fibrinogen. It is true that these substances the albuminoids of 

 milk, fibrinogen and egg white show an almost identical con- 

 stitution in various animal species, at least sufficient to render 

 them all susceptible to a given active serum irrespective of their 

 origin. 



5. The sensitizers of immune sera may often act on substances 

 other than those used for immunization, but from the same animal 

 species. For example, the serum of an animal injected with hen 

 blood acts on hen-egg white. There is no general rule in this respect, 

 however, as individual cases vary. 



